What is the F/M Ratio?

The Food-to-Microorganism (F/M) ratio is a critical process control parameter used in the operation of activated sludge wastewater treatment plants. It represents the relationship between the amount of food (organic matter measured as Biochemical Oxygen Demand or BOD) entering the system and the amount of microorganisms (measured as Mixed Liquor Volatile Suspended Solids or MLVSS) available to consume that food.

Maintaining the correct F/M ratio is vital for ensuring that the bacteria in the aeration tank remain healthy, settle well in the secondary clarifier, and effectively remove pollutants from the water. It is essentially a measure of how much you are "feeding" your microbial population relative to their population size.

How to Calculate F/M Ratio

The formula used by this calculator is based on the poundage of BOD divided by the poundage of MLVSS in the aeration tank:

F/M Ratio = (BOD, mg/L × Flow, MGD × 8.34) / (MLVSS, mg/L × Tank Volume, MG × 8.34)

The constant 8.34 is used to convert the units (mg/L and Million Gallons) into pounds. Since it appears in both the numerator and denominator, it cancels out mathematically, but it is traditionally used in field calculations to track actual pounds of organic load.

Typical F/M Ratio Ranges

The optimal F/M ratio varies depending on the specific design of the treatment plant:

• Conventional Activated Sludge: 0.2 to 0.5
• Extended Aeration: 0.05 to 0.15
• Completely Mixed Process: 0.2 to 0.6

Frequently Asked Questions

What happens if the F/M ratio is too high?

When the F/M ratio is too high, there is an excess of food relative to the number of microorganisms. This often leads to poor BOD removal, dispersed growth where bacteria do not clump together (flocculate), and cloudy effluent.

What happens if the F/M ratio is too low?

A low F/M ratio means the microorganisms are starving. This can lead to "old sludge," where the bacteria die off, creating "pin floc" that doesn't settle well, or encouraging the growth of filamentous bacteria which can cause sludge bulking.